Thermoplastic resin containers are excellent in shock resistance and the like and is easy to handle, and therefore, the increase of a demand for the container is also expected in the future. Especially, since containers made of a thermoplastic polyester resin such as polyethylene terephthalate are excellent in transparency, flavor properties, and heat resistance in addition to shock resistance, and they have gas barrier properties. Accordingly, these containers are broadly used as various kinds of containers.
As an example of such polyester containers, there is a container obtained by thermally molding an oriented or a non-oriented polyester resin sheet into a predetermined container shape such as a cup or tray shape. In general, after this type of container is packed with contents, an opening of the container is heat-sealed with a cover member.
Moreover, examples of a method for manufacturing this type of container include a method of drawing a softened polyethylene terephthalate sheet using a male mold plug to bring the sheet into contact by a female mold heated at a temperature which is not less than a glass transition point of the sheet; heat-setting the sheet; and allowing the sheet to shrink back on the male plug to cool and manufacture the container (see, e.g., Patent Document 1).
It is known that the thermoplastic polyester resin is subjected to a drawing step and a heat-setting step during molding to orientationally or thermally crystallize the resin, so that mechanical strength, transparency, and heat resistance are improved.
However, in a case where a portion along an opening rim part which is heat-sealed with the cover member is uniaxially oriented and crystallized in a diametric direction, transparency and heat resistance are obtained. However, this opening rim part is vulnerable to tension in a peripheral direction. When the container is dropped in a lateral direction, there is a problem that this opening rim part easily breaks, and sealability cannot be secured.
Moreover, when the polyester resin is orientationally or thermally crystallized, the heat sealability drops. Therefore, there is a problem that a heat sealing temperature needs to be remarkably raised, and a material to be used in the cover member is limited. The heat sealing time also needs to be lengthened, and there is a problem that productivity degrades during filling and sealing. Furthermore, since heat sealing strength itself is not improved, a heat seal portion might be peeled owing to dropping impact, and there is a problem that the heat sealing becomes difficult.
As a method of solving these problems, for example, in Patent Document 2, there is provided a method of irradiating the heat seal portion of the container with a laser beam to lower the crystallinity of the heat seal portion and to accordingly impart the heat-sealability thereto. According to this method, there is an effect of orientational crystallization or thermal crystallization, and the heat sealing is also possible.
Moreover, in this type of container, there is a demand for high sealing strength in a heat seal portion such that the cover member is not peeled even by a heating sterilization treatment or an inner pressure rise due to the dropping impact. For example, in Patent Document 3, a heat-sealed container is proposed in which a protruding portion is formed on a sealing face, and this protruding portion is integrated with the cover member in a case where a flange portion having the upward curved sealing face is heat-sealed with the cover member.
In the heat-sealed container constituted in this manner, after a root portion of the protruding portion is broken, the peeling of the sealing face proceeds. Therefore, initial seal breaking strength is improved up to the breaking strength of the protruding portion, and high sealing strength is realized in the heat seal portion.
On the other hand, the container proposed in Patent Document 3 is unfavorable from a viewpoint of easy opening properties of the cover member, because the protruding portion is also formed on an outer peripheral side of the sealing face. Therefore, various containers are also proposed in which, for example, a projecting portion having a asymmetric sectional shape is formed on a flange part, so that a resin lump piece functioning in the same manner as in the protruding portion in Patent Document 1 is selectively formed on an inner peripheral side of the sealing face during the heat sealing (e.g., Patent Documents 4, 5, 6, etc.).
In these containers, the easy opening properties are secured against a force to peel (open) the cover member inwards from the exterior of the container, so that the container is easily opened by a manpower owing to peeling of an interface between a sealant layer of the cover member and the flange part or cohesion failure of the sealant layer. On the other hand, the resin lump piece rises to prevent the heat seal portion from being peeled against a peeling force exerted outwards from the interior of the container, whereby high sealing strength is realized.
Patent Document 1: Japanese Patent Application Laid-Open No. 58-89319;
Patent Document 2: Japanese Patent Application Laid-Open No. 2-258577;
Patent Document 3: Japanese Patent Application Laid-Open No. 62-28355;
Patent Document 4: Japanese Patent Application Laid-Open No. 9-99933;
Patent Document 5: Japanese Patent Application Laid-Open No. 11-292140; and
Patent Document 6: Japanese Utility Model Application Laid-Open No. 6-44767.